1. Technical Field
The present invention is related to power amplifiers such as those used in cellular communications systems. More specifically, the present invention teaches a power amplifier having at least two output power devices and a mechanism for switching the output path between these two power devices. The first output power device is designed for power efficient signal amplification at the power amplifier""s highest output power level. The second output power device is designed for power efficient signal amplification at the output power level that the power amplifier is most likely to operate. By switching between the two power devices according to the output power level, a high level of efficiency can be achieved across a broad range of operating states of the power amplifier.
2. Background Art
The power efficiency of an amplification device such as a transistor varies with operating conditions. For example, a power amplifier designed for power efficiency at one supply voltage may well be inefficient at another supply voltage. However, applications abound that require the power amplifier to operate under varying conditions (e.g., different supply voltages) yet would benefit greatly from power efficient operation. In fact, while there are a variety of design constraints that may be imposed upon any power amplifier application, power efficiency is almost invariably a fundamental design objective.
By way of example, in most communication systems it is preferred that the power amplifier operate efficiently yet maintain an acceptable linearity over the desired supply voltage range. To that end, the typical power amplifier is adjusted to achieve a peak efficiency at a single output power level and supply voltage. In general, to operate at peak efficiency requires that the voltage swing at the output of the power amplifier be as large as possible. But, improving efficiency by increasing the voltage swing tends to reduce the linearity of the power amplifier. There is therefore a trade-off between power efficiency and linearity, improvements in one coming at the expense of the other.
Many cellular communication systems, such as the CDMA cellular system, require that the power amplifier deliver a wide range of output powers. For more details regarding CDMA cellular systems, please see the Electronic Industry Association""s publication EIA/TIA IS-95, which is incorporated herein by reference in its entirety. As will be appreciated, the power amplifier must safely operate at its highest power level. Being designed for the highest power level, the power amplifier tends to operate less efficiently at lower, more commonly used, power levels. Hence the life of a battery operated device is shortened because efficient power amplification is unavailable at the more commonly used power levels since the power amplifier must be designed for the highest power level.
Communication systems such as CDMA cellular systems merely illustrate one example of the multiplicity of power amplifier applications that require operation at numerous operating states. In order to address the shortcomings of the prior art, what is needed is a power amplifier that can achieve high power efficiency at several operating states.
In order to achieve the foregoing and in accordance with the present invention, a variety of power amplifiers having at least two output power devices and a mechanism for switching the output path between these two power devices are taught herein. The first output power device is designed for power efficient signal amplification at the power amplifier""s highest output power level. The second output power device is designed for power efficient signal amplification at the output power level that the power amplifier is most likely to operate. By switching between the two power devices according to the output power level, a high level of efficiency can be achieved across a broad range of operating states of the power amplifier.
A first embodiment of the present invention teaches a switchable path power amplifier suitable for amplifying an input signal received at a power amplifier input in order to generate an output signal at a power amplifier output. In this embodiment, the switchable path power amplifier has a first power device, a second power device, and a combining network device.
The first power device is intended for amplifying the input signal during a first operating state of the switchable path power amplifier and includes a first power device input coupled to the power amplifier input and a first power device output. The second power device is intended for amplifying the input signal during a second operating state of the switchable path power amplifier and includes a second power device input coupled to the power amplifier input and a second power device output. The combining network device includes a first combining network device input coupled to the first power device output, a second combining network device input coupled to the second power device output, and a combining network device output coupled to the power amplifier output. The combining network device is operable to select between the first power device and the second power device such that only one of the first power device and the second power device drives the power amplifier output.
In a related embodiment, the switchable path power amplifier further includes state determination circuitry operable to determine the operating state of the switchable path power amplifier. Typically, the state determination circuitry is arranged to control which power device drives the power amplifier output based upon the desired output power.
In a preferred embodiment, the combining network device includes an inductor L1 having first and second terminals, a capacitor C1 having first and second terminals, an inductor L2 having first and second terminals, a transmission line having first and second terminals, a capacitor C2 having first and second terminals, a capacitor C3 having first and second terminals, an inductor L3 having first and second terminals, and an inductor L4 having first and second terminals. These circuit components are connected as follows.
The first inductor L1 terminal is electrically coupled to the first power device output. The first inductor L2 terminal is coupled to a common ground reference, and the second inductor L2 terminal, the second inductor L1 terminal, and the first capacitor C1 terminal are coupled together. The first transmission line terminal, the second capacitor C1 terminal, and the power amplifier output are coupled together. The second capacitor terminal C2 is coupled to the common ground reference. The first capacitor C3 terminal, the first capacitor C2 termninal, and the second transmission line terminal are coupled together. The first inductor L3 terminal is coupled to the common ground reference. The first inductor L4 terminal is coupled to the second power device output. the second inductor L4 terminal, the second inductor L3 terminal, and the second capacitor C2 terminal are coupled together.
Yet another embodiment of the present invention teaches a switchable path power amplifier suitable for use in an RF communications system. The RF communication system has a first output power level that corresponds to the highest output power required of the RF communications system and a second output power level that corresponds to the output power at which the RF communications system most typically operates.
The switchable path power amplifier has a first power device substantially optimized for power efficient signal amplification at the first output power level, a second power device substantially optimized for power efficient signal amplification at the second output power level, state determination circuitry arranged to determine the power level at which the switchable path power amplifier is operating, and a combining network arranged to decouple a disabled power device from an output load coupled to the switchable path power amplifier such that any effect of the disabled power device upon an amplified electrical signal generated by the enabled power device is negligible. The state determination circuitry is further operable to a) enable the first power device and disable the second power device when the switchable path power amplifier is operating at the first output power level and b) enable the second power device and disable the first power device when the switchable path power amplifier is operating at the second output power level.
A related embodiment of the present invention contemplates a switchable path power amplifier having three or more power devices. In this embodiment, each of the three or more power devices would be suitably designed for a particular operating state (e.g., output power level). The state determination circuitry would therefore operate to enable the appropriate power device, and simultaneously disable to other power devices.
Another aspect of the present invention teaches a method for amplifying an electrical signal over multiple power levels, the amplified electrical signal driving an output load. The method begins by providing a switchable path power amplifier having first and second power devices, the first power device intended for amplifying the electrical signal during a first power level, and the second power device intended for amplifying the electrical signal during a second power level. The method then determines the power level of the switchable path power amplifier, and, when the power level is at the first power level, selects the first power device for use in amplifying the electrical signal. However, when the power level is at the second power level, the method selects the second power device for use in amplifying the electrical signal.